Development of alternative fuels has gained increasing importance with increased awareness of environmental concerns and in the wake of economic pressures and fuel costs that have resulted from supply volatility of petroleum and other fossil-based resources. Policy makers have recognized this importance and have encouraged production and increased use of biofuels. As interest has grown in moving away from petrochemical or natural gas-derived hydrocarbon sources, some have concentrated on finding renewable and sustainable “green” carbon resources. Early so-called first generation biofuels are made by fermenting food products (e.g., starch-containing and sugary plants, such as corn, wheat, or sugar cane) into short-chain alcohols (e.g. ethanol, butanol) or transesterifying fatty acids liberated from plant oils by glycerolysis (e.g., rape seed, soybeans) to make fatty acid methyl esters used as biodiesel. (See e.g., Cesar B. Granda et al., “Sustainable Liquid Biofuels and Their Environmental Impact,” ENVIRONMENTAL PROGRESS, Vol. 26, No. 3, pp. 233-250 (October 2007), incorporated herein by reference.)
Carboxylic acids such as used to make biodiesel can also be made by fermentation from carbohydrate sources, in addition to being derived from plant oils. Although the fermentative production of carboxylic acids, such as malic or succinic acid, has several advantages over petrochemical-based fuels, the production costs for the bio-based carboxylic acids have been too high for bio-based production to be cost-competitive with petrochemical production regimes. (See e.g., James McKinlay et al., “Prospects for a Bio-based Succinate Industry,” APPL. MICROBIOL. BIOTECHNOL., (2007) 76:727-740; incorporated herein by reference.) In fermentation of starches and sugars, carboxylic acids are generated using microorganisms. Microorganisms require certain conditions for their functions. For example, with most commercially viable succinate producing microorganisms described in the literature, one needs to neutralize the fermentation broth to maintain an appropriate pH for maximum growth, conversion and productivity.
Currently, the carboxylic acids are recovered from fermentation broths as salts instead of as free acids. Typically, the pH of the fermentation broth is maintained at or near a pH of 7 by introduction of ammonium hydroxide or other base into the broth, thereby converting the carboxylic acid into the corresponding acid salt. Generation of carboxylic acid salts versus the free organic acid form brings about significant processing costs. About 60% of the total production costs are generated by downstream processing, because of the difficulties associated with the separation and purification of the acids and their salts in the fermentation broth.
Recovery of carboxylic acids as salts has a number of associated problems and requires several different steps in post-fermentation, downstream processing to isolate free acids and to prepare the carboxylic acids for chemical transformation and to convert the raw acids to useful compounds. When salts are generated in conventional fermentation processes, an equivalent of base is required for every equivalent of acid to neutralize. The amount of reagent used can increase costs. Further, one needs to remove the counter ions of the salts so as to yield free acids, and one needs to remove and dispose of any resulting waste and by-products. For instance, calcium salts of C4 diacids have a very low solubility in aqueous broth solutions (typically less than 3 g/liter at room temperature), and are not suitable for many applications for which a free acid species is needed, such as chemical conversion to derivative products like butanediol and biofuels. Therefore, the calcium salt is typically dissolved in sulfuric acid, forming insoluble calcium sulfate, which can readily be separated from the free diacid. Calcium sulfate is a product having few commercial applications, and accordingly is typically discarded as a solid waste in landfills or other solid waste disposal sites. All of these individual operational units contribute to the overall costs of the process.
To reduce waste and costs associated with generating free carboxylic acids and to improve the recovery yield, a need exists for a better, more direct method of recovering a variety of carboxylic acids, such as malic or succinic acid, and which can provide a successful route to combine a biologically-derived hydrocarbon feedstock with the production of various biofuel products, such as ethane, propane, propanol or butanol, by means of hydrogenation or hydrogenolysis. Such a streamlined, green process would be a welcome innovation.